Photoresists are photosensitive films for transfer of images to a substrate. They form negative or positive images. After coating a photoresist on a substrate, the coating is exposed through a patterned photomask to a source of activating energy, such as ultraviolet light, to form a latent image in the photoresist coating. The photomask has areas opaque and transparent to activating radiation that define an image desired to be transferred to the underlying substrate.
Chemical amplification-type photoresists have proven to be useful in achieving high sensitivity in processes for forming ultrafine patterns in the manufacture of semiconductors. These photoresists are prepared by blending a PAG with a polymer matrix having acid labile structures. According to the reaction mechanism of such a photoresist, the photoacid generator generates acid when it is irradiated by the light source, and the main chain or branched chain of the polymer matrix in the exposed or irradiated portion reacts in a so called “post exposure bake” (PEB) with the generated acid and is decomposed or cross-linked, so that the polarity of the polymer is altered. This alteration of polarity results in a solubility difference in the developing solution between the irradiated exposed area and the unexposed area, thereby forming a positive or negative image of a mask on the substrate. Acid diffusion is important not only to increase photoresist sensitivity and throughput, but also to limit line edge roughness due to shot noise statistics.
In a chemically amplified photoresist, the solubility-switching chemistry necessary for imaging is not caused directly by the exposure; rather exposure generates a stable catalytic species that promotes solubility-switching chemical reactions during the subsequent PEB step. The term “chemical amplification” arises from the fact that each photochemically-generated catalyst molecule can promote many solubility-switching reaction events. The apparent quantum efficiency of the switching reaction is the quantum efficiency of catalyst generation multiplied by the average catalytic chain length. The original exposure dose is “amplified” by the subsequent chain of chemical reaction events. The catalytic chain length for a catalyst can be very long (up to several hundred reaction events) giving dramatic exposure amplification.
Chemical amplification is advantageous in that it can greatly improve resist sensitivity, but it is not without potential drawbacks. For instance as a catalyst molecule moves around to the several hundred reactions sites, nothing necessarily limits it to the region that was exposed to the imaging radiation. There is a potential trade-off between resist sensitivity and imaging fidelity. For example, the amplified photoresist is exposed through a photomask, generating acid catalyst in the exposed regions. The latent acid image generated in the first step is converted into an image of soluble and insoluble regions by raising the temperature of the wafer in the PEB, which allows chemical reactions to occur. Some acid migrates out of the originally exposed region causing “critical dimension bias” problems. After baking, the image is developed with a solvent. The developed feature width may be larger than the nominal mask dimension as the result of acid diffusion from exposed into the unexposed regions. For much of the history of amplified resists this trade-off was of little concern as the catalyst diffusion distances were insignificant relative to the printed feature size, but as feature sizes have decreased, the diffusion distances have remained roughly the same and catalyst diffusion has emerged as a significant concern.
In order to generate enough acid which would change the solubility of the polymer, a certain exposure time is required. For a known PAG molecule like N-Hydroxynaphthalimide triflate (“NIT”), this exposure time is rather long (due to its low absorption at 365 nm or longer). Increasing the concentration of such PAGs, however, will not result in faster exposure times because the solubility of the PAG is the limiting factor. Another possibility is to add sensitizers which absorb the light and transfer energy to the PAG which would then liberate the acid. Such sensitizers, however, must be used in rather high concentrations in order to be able to transfer the energy to a PAG in close proximity. At such high concentrations, sensitizers often have an absorption which is too high and has negative effects on the shape of the resist profile after development.
Accordingly, there is a need in the art for PAGs that exhibit better a solubility, which means that more active molecules are imparted into the formulation, wherein a photoresist composition comprising these compounds has a high sensitivity towards electromagnetic radiation, in particular towards electromagnetic radiation with a wavelength of 200 to 500 nm, and—at the same time—allows the production of a patterned structure with a higher resolution, compared to the photoresist compositions known from the prior art.